1. Field of the Invention
This invention relates generally to oxidant feed tube supports for tubular solid oxide fuel cells (SOFCs) disposed in a fuel cell generator.
2. Background Information
High temperature, solid oxide electrolyte fuel cell generators, which are made of mostly ceramic components, and which allow controlled leakage among plural chambers in a sealed housing, are well known in the art, and are taught in U.S. Patent Nos. 4,395,468 and 5,573,867 (Isenberg and Zafred, et al., respectively). One type of such prior art generator design 12 is shown in FIG. 1 of the present application (which corresponds to FIG. 4 of the Zafred et al. patent). Here, oxidant gas enters air manifold 48 through oxidant inlet lines 20 as oxidant stream 50 (usually air), passing upward, possibly through cooling ducts, to a top air distribution plenum 52. The oxidant stream 50xe2x80x2 is then transferred and passes downward via individual thin, ceramic oxidant feed tubes 51. The oxidant passes down the feed tubes 51 into the bottom interior of each fuel cell 36, where, as is well known in the art, the oxidant reverses flow and passes up the annular space between the oxidant feed tube and the interior air electrode, where it reacts along the air electrode interior surface. The reacted oxidant, in one embodiment of the fuel cell generator, finally enters a combustion chamber section 54 as spent oxidant, where the spent oxidant then combusts with spent fuel and the remaining unreacted feed fuel, to provide exhaust gas 56, which flows to an exhaust manifold. Part of the spent fuel may also be recirculated to the ejector 40, as shown in FIG. 1. The exhaust gas 56 passes through exhaust ducts 58. A power lead 84 is also shown in FIG. 1, and fuel inlet is shown as piping 38 which passes through prereformer 42.
The tubular fuel cells include a solid oxide electrolyte sandwiched between two porous ceramic electrodes, an outer fuel electrode and an inner air electrode. FIG. 2 of the present application shows a prior art oxidant/air feed tube conduit support system employed in the Isenberg patent (shown in FIG. 2 of Isenberg). A metal tube sheet 34 has associated bores 60 that fit loosely around the oxidant feed tube conduits 51 to allow free thermal expansion. The oxidant feed tube conduits 51 are comprised of alumina or alumina compounds, and the tube sheet is covered with an insulation 62 such as low density alumina. Leakage of oxidant, into the combustion (pre-heating) chamber 16, as indicated by arrow 63 in FIG. 2, was considered acceptable. The oxidant feed tubes then proceed into the interior of the tubular fuel cells, as shown in FIG. 1 of the present application.
A later oxidant/air feed tube conduit support system was taught by Draper et al., in U.S. Pat. No. 4,664,986, and also in U.S. Pat. Nos. 4,808,491 and 4,876,163 (both Reichner). The prior art system of Draper et al. taught welding the air feed tube conduits to associated subheader tubes, so that there was substantially no air leakage. In the prior art Reichner system, as shown in FIG. 3 of the present application (and FIG. 1B of the Reichner ""491 patent), oxidant/air feed 50 flows into top oxidant/air distribution plenum 52 and then into further oxidant/air distribution plenums 52xe2x80x2, where the oxidant/air then passes downward via individual oxidant feed tubes 51. At the top of the oxidant feed tubes 51 spherical supports 70 kept the oxidant feed tubes in place. These spherical supports required a machined spherical seat 72 in the Inconel plenum wall 74 at the bottom of the plenums 52xe2x80x2. Insulation 76 surrounds the plenums. Steel enclosure 78 surrounds the fuel cell generator. Exhaust gas passages are shown as channels 80 and the bottom lower plenum enclosure insulation board is shown as 82 supporting the bottom of plenum 52xe2x80x2. The Isenberg design (as shown in FIG. 2 of this application) required precise cutting of the insulation layer and after thermal cycling, the possibility that the air feed tube would slip out of place to contact the bottom of the fuel cell cutting off the air supply for that cell. The Draper et al. design was very expensive and very heavy and required major machining and welding of Inconel components. The Reichner design (shown as FIG. 3 of the present application) also required substantial machining to properly set the spherical support. Also, in current SOFC design, a perfectly centered 72 in. (182.8 cm) long air feed tube in a 66 in. (167.6 cm) long cell permits only 0.1xc2x0 freedom of movement before it closes the 0.125 inch (0.32 cm) radial gap and hits the inside surface of the cell. This radial gap is further reduced if there is any bow in the cell or the air feed tube. The difficulty of assembling the air feed tube in a cell is further compounded by any cell-to-cell and bundle-to-bundle lateral and torsional misalignment resulting from corresponding sintering operations. Due to the lack of lateral freedom and the extremely limited angular movement of the air feed tubs, there is a great likelihood of wedging the air feed tube into the cell wall during assembly if there is any distortion (lateral, torsional, or bow) of the cell.
What is needed is an improved, simpler, less expensive oxidant/air feed tube conduit support system that will require only minimal or no metal finishing. Therefore, it is one of the main objects of this invention to support the oxidant/air feed tubes in such a way as to keep them from contacting the sides or bottom inside of the fuel cells. It is another main object to provide a simpler, significantly less expensive oxidant/air feed tube support system which requires minimal or no metal finishing. It is also an object of this invention to provide a minimum leakage seal between the air plenum and the rest of the SOFC module and to bring the design to a commercialization level.
These and other objects of the invention are accomplished by providing a solid oxide fuel cell generator comprising: hollow, tubular fuel cells with interior air electrodes; an oxidant plenum having a lower enclosing member; and oxidant feed tubes; where the enclosing member contains a plurality of holes, and supports the oxidant feed tubes, which pass from the oxidant plenum into the center of the fuel cells through the holes in the enclosing member; and where a compliant gasket around the top portion of the oxidant feed tubes and on top of the enclosing member helps center the oxidant feed tubes within the fuel cells. The compliant gasket provides a cushion during shipping and during generator use. The gasket is held on by friction, no glue being necessary.
The invention also resides in a solid oxide fuel cell generator comprising: (1) a plurality of electrically connected hollow, tubular, solid oxide fuel cells which can operate on feed fuel and oxidant gases; (2) an oxidant plenum within the generator for receiving and distributing fed oxidant to the fuel cells; (3) a lower enclosure for the oxidant plenum having a plurality of holes therethrough; and (4) a plurality of hollow oxidant feed tubes extending from the oxidant plenum, through the lower oxidant plenum enclosure holes into the inside of the fuel cells where the lower oxidant plenum enclosure has at least a flat top surface facing the oxidant plenum and supports the oxidant feed tubes by contact with a washer and gasket tube support combination, which combination is disposed around the outer circumference of the oxidant feed tubes and on top of the flat top surface of the oxidant plenum enclosure;. where the tube support combination consists of a top, hard washer bonded to the oxidant feed tubes and a compliant gasket disposed on top of and contacting the flat top surface of the lower oxidant plenum enclosure between the washer and the lower oxidant plenum enclosure, and where at least the gasket has a bottom surface which mates to the flat top surface of the lower oxidant plenum enclosure. Preferably the gasket will completely cover the lower oxidant plenum enclosure holes, which can be oversized, by as much as 20% to 30% to allow adjustment of the feed tube positioning to provide good alignment of the oxidant feed tubes to center them within the inside of the hollow fuel cells. The dense washer is preferably attached to the oxidant feed tubes and helps compress the compliant gasket and prevent the feed tubes from sliding or slipping through the holes in the enclosing member.
Among the important advantages of this design: it permits an increase in the plenum hole diameter, thus providing the needed freedom for increased lateral and rotational alignment of the air feed tube with the cell at assembly; it reduces the support system cost by a factor of 6 to 10; it eliminates the costly spherical seal machining in the air plenum, further reducing the plenum cost; and the ability to increase the plenum hole diameter permits relaxing the feed tube bow requirements over the full length, thereby significantly reducing the rejection rate and reducing the cost of the air feed tube by a factor of 8 to 10. The cost of the air feed tube is dramatically reduced because key manufacturing tolerances can be relaxed, thereby lowering reject rates.